Superdiffusion of energetic particles at shocks: A L\'evy Flight model for acceleration

TL;DR

This paper extends a Le9vy flight model to study superdiffusive particle transport at shocks, revealing faster acceleration and harder spectra compared to Gaussian diffusion, with implications for understanding energetic particle behavior in the Heliosphere.

Contribution

It introduces a Le9vy flight-based model for superdiffusive transport at shocks, comparing its effects on acceleration time and spectral slope to traditional diffusion models.

Findings

Superdiffusive transport results in harder energy spectra at shocks.

Superdiffusive transport accelerates particles faster than Gaussian diffusion.

Le9vy flight models reproduce observed features in the Heliosphere.

Abstract

In the Heliosphere, power-law particle distributions are observed e.g. upstream of interplanetary shocks, which can result from superdiffusive transport. This non-Gaussian transport regime may result from intermittent magnetic field structures. Recently, we showed that a L\'evy flight model reproduces the observed features at shocks: power-law distributions upstream and enhanced intensities at the shock. We extend the L\'evy flight model to study the impact of superdiffusive transport on particle acceleration at shocks. The acceleration time scale and spectral slope are compared to Gaussian diffusion and a L\'evy walk model. The fractional transport equation is solved by sampling the number density with the corresponding stochastic differential equation that is driven by an alpha-stable L\'evy distribution. For both Gaussian and superdiffusive transport we use a modified version of CRPropa 3.2. We obtain the number density and energy spectra for constant and energy-dependent anomalous diffusion and find, compared to the case of Gaussian diffusion, harder energy spectra at the shock as well as faster acceleration. The spectral slope is even harder than predicted for L\'evy walks. L\'evy flight models of superdiffusive transport lead to observed features in the Heliosphere. We further show that superdiffusive transport impacts the acceleration process by changing the probability to escape the shock. The flexibility of the L\'evy flight model allows for further studies in the future, taking the shock geometry and magnetic field structure into account.